Field of the Invention
The present invention relates to a method of controlling the output power of a power amplifier, and a device therefor.
Control devices for automatically controlling the output of a power amplifier using a feedback scheme have been proposed in various forms in the past. This type of control device detects power output from a power amplifier, compares it with a reference value, and controls the gain of the amplifier such that the difference between the detected power output and the reference value is reduced to zero or to a particular value. As a result, the output power of the amplifier is automatically controlled to be at a predetermined target value. Such a control device is disclosed in Japanese Utility Model Laid-Open Publication No.-5-9018 by way of example.
The conventional feedback type control device is implemented as a so-called closed loop. Hence, when the gain of the loop is increased in order to enhance the response of the control over the output voltage, the stability of the control loop is deteriorated, a result which is well understood from the general theory of control systems. Consequently, the power amplifier output becomes unstable even when the predetermined output must be maintained. Moreover, in practice, the closed loop each has a frequency characteristic based on the frequency response of its constituent parts and is apt to oscillate when the loop gain is greater than 1. This frequency characteristic also causes the phase to be inverted. The increase in loop gain aggravates the probability of oscillation. Therefore, the loop gain cannot be increased beyond a certain limit, resulting in slow control response.